To the general public, the organism known as Propionibacterium acnes (P acnes) is the pathogen that causes acnes vulgaris, a skin condition that results in ugly papules and permanent scarring. The media perception of acne infection is focused on these scarring effects, which are magnified by the social implications of such a disease.

In the medical community, however, this organism is gaining significant notice for its devastating effects on deeper organ systems. It is recognized as the primary pathogen in infections after shoulder surgery and has been found to infect the brain, cranium, and spine as well. The implications of P acnes are even expanding into the realm of osteoarthritis as orthopaedic researchers examine whether it plays a role in advancing joint degeneration.

Less well-known is this organism’s role as healthy skin bacterial flora in up to 70 percent of humans. The main issue with detecting when P acnes becomes a pathogen is its notoriously indolent growth characteristics. It doesn’t grow in the same way that conventional bacteria such as Staphylococcus and Streptococcus do; rather, it generates little inflammation, grows slowly in the deep tissues, and avoids detection in standard wound cultures.

Common inflammatory markers drawn from blood tests such as C-reactive protein and erythrocyte sedimentation rates fail to rise consistently with this type of infection. Multiple reports over the past 15 years by experienced shoulder surgeons such as John W. Sperling, MD, of the Mayo Clinic, have shown that P acnes infections may develop anywhere from 3 months to 2 years after surgery.

These infections are only recognized if the surgeon specifically asks for the organism to be cultured on special medium, in special conditions, and for a duration 3 times the normal. Cultures should be plated on Chocolate agar medium in an anaerobic environment for 21 days.

These facts beg the question: If you are not looking, how will you find it?

More questions than answers
The orthopaedic surgery community has been actively researching this quandary. As with most scholarly pursuits, the more that is known, the more questions that are posed. P acnes is described as the most common infectious agent after shoulder replacement surgery, causing loosening and persistent pain. Even in minimally invasive procedures of the shoulder such as shoulder arthroscopy, 50 percent to 86 percent of postoperative infections are attributed to P acnes. However, in total this rate is still well below 1 percent of all surgeries performed.

Most recently, Prof. Ofer Levy, MD, MCh(Orth), FRCS, at Royal Birkshire Hospital, United Kingdom, has investigated this subject and found P acnes infections in more than 40 percent of shoulder joints requiring shoulder replacement due to severe arthritis. As both P acnes infection and progressive osteoarthritis result in a slow degradation of cartilage within the joint, it is not unimaginable that shoulder infections in the past may have been missed and attributed to progressive osteoarthritis.

In our personal experience in southern California, we have seen patients with P acnes infection in multiple presentations. One example, an 85-year-old gentleman, stands out. He underwent a shoulder arthroscopy procedure many years before he came to our office with a football-sized cyst from his neck to his shoulder. Radiographs revealed advanced erosion of the entire glenohumeral and acromioclavicular joints, as if the shoulder had melted like hot wax around a malformed eroding ball. The cyst had been fed by the bacterial breakdown of the multiple joints within his shoulder and slowly expanded to envelope his shoulder girdle. Only through multiple biopsies and cultures of the cyst were we able to implicate P acnes infection as the cause of this dramatic case.

As part of a recent clinical study, we have analyzed cultures from inside the shoulder joint after all shoulder arthroscopies and have found P acnes within the joint in one-fifth of cases. Although we cannot determine whether the organism spontaneously migrated into the deep tissues or was placed there through our actions (a more likely scenario), we have changed our shoulder surgery preparation, diagnosis, and treatment protocols.

Preventive measures include a skin pre-preparation with benzoyl peroxide 10 percent wash before standard preparation. Two weeks of treatment with tetracycline, penicillin, and later generation cephalosporin antibiotics has been shown to be the most effective regimen with the least chance of resistance.

Although it is premature to draw parallels between the role of P acnes in osteoarthritis and the description of Helicobacter pylori in gastric pathology, physicians in general should be alert to the unique presentations of this bug. The general fear among orthopaedic practitioners is that by expanding the indications for screening, more cases will be found, but how cost-effective would regular screening be? The answer is unknown; however, it seems wise to address the possibility of P acnes infection in the cases of failed shoulder arthroplasty and arthroscopy procedures.

Michael J. Chuang, MD, completed fellowship training in arthroscopic surgery and sports medicine at The Sports Clinic in Laguna Hills, Calif. Wesley M. Nottage, MD, is in private practice at The Sports Clinic in Laguna Hills, Calif.

Bottom Line

• P acnes is the most common cause for postoperative shoulder infection following arthroplasty and arthroscopy and occurs 3 to 24 months after surgery.
• Prolonged, persistent pain or poor clinical outcomes after shoulder surgery should raise concerns about P acnes infection.
• Inflammatory markers and regular infectious clinical signs are extremely unreliable for diagnosing this infection.

References:

1. Levy O, Iyer S, Atoun E, et al: Propionibacterium acnes: An underestimated etiology in the pathogenesis of osteoarthritis? J Shoulder Elbow Surg 2013;22(4):505-511. doi: 10.1016/j.jse.2012.07.007. Epub 2012 Sep 13.
2. Singh JA, Sperling JW, Schleck C, Harmsen WS, Cofield RH: Periprosthetic infections after total shoulder arthroplasty: A 33-year perspective. J Shoulder Elbow Surg 2012;21(11):1534-1541. doi: 10.1016/j.jse.2012.01.006. Epub 2012 Apr 18.
3. Singh JA, Sperling JW, Schleck C, Harmsen W, Cofield RH. Periprosthetic infections after shoulder hemiarthroplasty. J Shoulder Elbow Surg 2012 Oct;21(10):1304-1309. doi: 10.1016/j.jse.2011.08.067. Epub 2011 Dec 11.